Electroluminescent devices are generally classified into inorganic electroluminescent devices that use inorganic compounds in the light-emitting devices and organic electroluminescent devices that use organic compounds in the light emitting devices. In recent years, studies are underway with emphasis laid on putting organic electroluminescent devices to practical use on account of their distinctive feature of emitting light of high luminance at low voltage.
An organic electroluminescent device is basically constructed by forming a hole-injecting layer on a glass plate on which a thin film of an anode material such as indium tin oxide (ITO) is vapor-deposited, then forming a thin film of an organic layer such as a light-emitting layer on the hole-injecting layer, and finally forming a thin film of a cathode material on the organic layer. Some devices are constructed by suitably adding a hole-transporting layer or an electron-transporting layer to this basic structure. Examples of the layered structure of an organic electroluminescent device include anode/hole-injecting layer/light-emitting layer/electron-transporting layer/cathode and anode/hole-injecting layer/hole-transporting layer/light emitting layer/electron-transporting layer/cathode.
Light-emitting materials to be used in the light-emitting layer of an organic electroluminescent device are roughly classified into low-molecular-weight light-emitting materials and high-molecular-weight light-emitting materials.
A low-molecular-weight light-emitting material is formed into a film for the light-emitting layer mainly by the vacuum vapor deposition process and this film-forming process is characterized in that a variety of materials having different functions can be readily piled one upon another to form a multilayer structure and this enables fabrication of an organic electroluminescent device of high performance. However, as the display panels become larger in size and finer, this process faces problems in that a uniform control of the film thickness becomes difficult to exercise and color separation becomes difficult to perform and, besides, the production cost rises as the process requires a large-scale vacuum apparatus.
Studies are also underway to develop a practical film-forming process based on solution coating for forming a light-emitting layer from a low-molecular-weight light-emitting material. However, segregation and phase separation are observed as a low-molecular-weight compound crystallizes on application of this technique and further improvements are needed to achieve a level of practical use.
On the other hand, since the vacuum vapor deposition process is not applicable as a film-forming process to practically all of high-molecular-weight light-emitting materials, a process based on solution coating such as spin coating, printing, and ink-jet coating is used instead. This process is readily applicable to large-size panels and excellently suited to mass production; however, the problem this process faces is that coating solutions are difficult to apply layer by layer to form a laminated film and contaminants enter with ease. For this reason, devices using high-molecular-weight light-emitting materials are inferior to devices using low-molecular-weight light-emitting materials in properties such as luminous efficiency and lifetime. Under these circumstances, a demand has been created for high-molecular-weight light-emitting materials having excellent luminous properties and good film-formability.
In attempts to meet the aforementioned property requirements, patent documents 1 and 2 and non-patent document 1 report that utilization of phosphorescence enables fabrication of organic electroluminescent devices of high luminous efficiency. However, the organic electroluminescent devices disclosed in these documents display luminous efficiency and stability as insufficient as those of ordinary fluorescent electroluminescent devices and they fail to make sufficient improvements.
Further, as a means to enhance the luminous efficiency of an organic electroluminescent device, patent documents 3 and 4 disclose a polymer light-emitting material comprising a metal complex moiety as a phosphorescent dopant obtained by metal complexation of a ligand moiety partly incorporated in the backbone or side chain of a polyarylene which is a π-conjugated polymer and also disclose a light-emitting device using the said polymer light-emitting material. However, this material cannot yield a light-emitting device of high luminous efficiency because the metal complexation is not sufficient to provide functionality as a phosphorescent dopant and the phosphorescence quantum efficiency ascribable to the π-conjugation system of the polymer is low. Further, π-conjugated polymers are poorly soluble in organic solvents and they are not suitable for use in film forming by a solution coating process.
Patent document 5 discloses a polymer light-emitting material of high solvent solubility and large phosphorescence energy comprising a metal complex moiety as a phosphorescent dopant incorporated by metal complexation of a ligand moiety partly introduced to the backbone of polyethylene. Although the phosphorescent luminous efficiency is expected to be enhanced by introduction of the polyethylene backbone, the metal complexation is not sufficient to provide functionality as a phosphorescent dopant and it cannot be said that the anticipated performance is fully displayed.
Further, patent documents 6 and 7 disclose the use of polymerization or copolymerization of a polymerizable phosphorescent dopant compound (an iridium complex) to produce a polyethylene resin in which iridium complexes are linked to the backbone as a polymer light-emitting material. However, this synthetic method yields a polymer containing blocks of the iridium complex thereby creating a condition under which the concentration of light-emitting excitons becomes too high locally or a condition of the so-called concentration quenching or concentration deactivation and lowering the luminous efficiency. Hence, it is difficult to raise the concentration of the iridium complex. Furthermore, a vinyl compound to which an iridium complex is linked shows low solubility in a polymerization reaction solvent and it is difficult to raise the concentration of the iridium complex in the polymer. This in turn makes it difficult to form a film containing a sufficient concentration of the iridium complex, that is, it is difficult to say that sufficient light-emitting performance can be extracted by this method.    Patent document 1: JP H8-319482 A    Patent document 2: JP H11-256148 A    Patent document 3: JP2003-73479 A    Patent document 4: JP2003-73480 A    Patent document 5: JP2002-293830 A    Patent document 6: JP2003-119179 A    Patent document 7: JP2006-008996 A    Non-patent document 1: Appl. Phys. Lett., 77, 904 (2000)